Peginterferon alfa-2a
Pronunciation: pronunciation varies by local usage.
- Generic name
- peginterferon alfa-2a
- Brand names
- Pegasys
- Drug class
- Pegylated interferon alfa-2a
- Form
- Injection
- Route
- Subcutaneous injection
- Legal status
- Prescription-only biologic medicine
Availability and supply rules vary by country
Trade names by country: United States · Canada
Australia: · Full directory
What peginterferon alfa-2a is and how it works
Peginterferon alfa-2a is a pegylated alpha interferon used to treat chronic hepatitis B and chronic hepatitis C in eligible patients. It is given by subcutaneous injection once weekly. (1)
It is used in adults with compensated liver disease, and in children for certain hepatitis B and C indications. Treatment is managed by a specialist; it is not a cure and requires monitoring for serious side effects.
Mechanism of action: how peginterferon alfa-2a works
Peginterferon alfa-2a is a covalent conjugate of recombinant interferon alfa-2a with a branched bis-monomethoxy polyethylene glycol moiety, prolonging its half-life for once-weekly dosing. It exerts antiviral and immunomodulatory effects. (1)
What peginterferon alfa-2a is used for
Chronic hepatitis B
For adults with HBeAg-positive or HBeAg-negative chronic hepatitis B who have compensated liver disease, and for non-cirrhotic children 3 years and older with HBeAg-positive chronic hepatitis B. Treatment is 48 weeks. (1)
Chronic hepatitis C
For adults with chronic hepatitis C and compensated liver disease, used in combination with other antiviral medicines; monotherapy only when other HCV medicines are contraindicated. Also for children 5 years and older in combination with ribavirin. (1)
Before taking peginterferon alfa-2a
Do not take / not appropriate for
- Do not receive peginterferon alfa-2a if you have autoimmune hepatitis. (all indications) (1)
- Do not receive peginterferon alfa-2a if you have hepatic decompensation (Child-Pugh class B or C). (all indications) (1)
- Neonates and infants should not receive this medicine because it contains benzyl alcohol. (all indications) (1)
- Do not receive peginterferon alfa-2a if you have had a severe allergic reaction to it or any interferon. (all products) (1)
- If peginterferon alfa-2a is combined with ribavirin, do not use this combination if you are pregnant or if your male partner is pregnant. (combination with ribavirin) (1)
Tell your clinician about
- Any history of depression, suicidal thoughts, mental illness, or substance use.
- Heart disease, high blood pressure, chest pain, or irregular heartbeat.
- Autoimmune disease (e.g., lupus, rheumatoid arthritis), thyroid problems, or diabetes.
- Kidney, liver, or eye problems.
- Pregnancy, plans for pregnancy, breastfeeding, or if you are a male patient whose partner could become pregnant.
- All current medicines, especially theophylline, methadone, azathioprine, and hepatitis nucleoside analogues.
How to take peginterferon alfa-2a
- Peginterferon alfa-2a is injected under the skin (thigh or abdomen) once weekly.
- The dose is determined by the clinician; for adults, usually 180 mcg; for children, the dose is calculated based on body surface area (up to 180 mcg).
- Treatment duration is prescribed: hepatitis B for 48 weeks; hepatitis C as recommended in the regimen.
- Do not change the dose, stop treatment, or switch to another interferon product without clinician advice.
- Follow proper storage and administration instructions; dispose of needles safely.
Procedures and treatment changes
Tell the treating team before surgery, procedures, or any medicine change. Do not alter treatment without the responsible clinician. (1)
Missed dose
Contact your healthcare provider for instructions if you miss a dose. Do not take an extra dose to make up for a missed one.
Overdose
Contact your healthcare provider or poison control center immediately if you suspect an overdose. Symptoms may include severe worsening of side effects. (1)
Side effects
Peginterferon alfa-2a can cause serious side effects, including neuropsychiatric, autoimmune, ischemic, and infectious disorders. Common side effects include fatigue, fever, headache, and muscle aches. This list is not exhaustive; report any new or concerning symptoms.
Flu-like symptoms
What to notice: Fatigue, fever, muscle aches, headache, chills.
What to do: These are common, especially early in treatment. Tell your clinician if they become severe or persistent. (1)
Blood count changes (neutropenia, anemia)
What to notice: Increased risk of infection (fever, sore throat), unusual bleeding or bruising, pale skin, extreme tiredness.
What to do: Regular blood tests will monitor your blood counts. Report signs of infection or bleeding promptly. (1)
Injection-site reactions
What to notice: Redness, pain, swelling, or itching at the injection site.
What to do: These usually improve. Inform your clinician if severe, spreading, or accompanied by skin breakdown. (1)
Mood changes and sleep disturbances
What to notice: Feeling depressed, anxious, irritable, or having insomnia.
What to do: Report any mood changes, especially suicidal thoughts, to your prescriber immediately. (1)
Hair thinning (alopecia)
What to notice: Noticeable hair loss or thinning.
What to do: This may occur. Discuss concerns with your clinician. (1)
Loss of appetite (anorexia)
What to notice: Decreased appetite, weight loss.
What to do: Monitor weight and nutrition. Talk to your clinician if significant. (1)
New or worsening depression or suicidal thoughts
What to notice: Feeling very sad, hopeless, losing interest in activities, thoughts of self-harm or harming others.
What to do: Contact your prescriber immediately. These can be life-threatening and may require stopping treatment. (1)
Chest pain or heart problems
What to notice: Chest pain, pressure, tightness, irregular heartbeat, shortness of breath, fainting.
What to do: Seek emergency medical attention. (1)
Severe abdominal pain
What to notice: Intense stomach pain, bloody diarrhea, painful bowel movements.
What to do: Contact your clinician promptly; these may indicate colitis or pancreatitis. (1)
Vision changes
What to notice: Blurred vision, loss of vision, floaters, or other eye symptoms.
What to do: Get an eye examination promptly; untreated eye problems can cause permanent vision loss. (1)
Signs of severe infection or bleeding
What to notice: Fever, chills, sore throat, unusual bruising or bleeding, dark stools.
What to do: Contact your clinician; blood counts may be dangerously low. (1)
Liver problems
What to notice: Yellowing of skin or eyes, dark urine, severe nausea, vomiting, confusion (hepatic encephalopathy).
What to do: Contact your clinician immediately; treatment may need to be discontinued. (1)
Breathing problems
What to notice: New or worsening cough, shortness of breath, chest pain with breathing, wheezing.
What to do: Contact your clinician promptly; lung problems such as pneumonia or pulmonary hypertension may occur. (1)
Thyroid dysfunction
What to notice: Symptoms of overactive thyroid (rapid heartbeat, sweating, weight loss) or underactive thyroid (fatigue, weight gain, feeling cold).
What to do: Your thyroid function will be monitored. Report these symptoms to your clinician. (1)
Blood sugar changes
What to notice: Excessive thirst, frequent urination, confusion, or fainting (high or low blood sugar).
What to do: Report these symptoms; blood sugar monitoring may be needed, especially in people with diabetes. (1)
Serious warnings
Neuropsychiatric disorders — emergency
Symptoms: Severe depression, suicidal or homicidal ideation, aggressive behavior, psychosis, hallucinations.
Action: Contact your prescriber immediately. These reactions can be life-threatening and may require immediate discontinuation of the medicine. (1)
Hepatic decompensation — emergency
Symptoms: Worsening liver function, ascites (fluid in abdomen), jaundice, confusion (hepatic encephalopathy), bleeding problems.
Action: Seek emergency medical care. The clinician will discontinue the medicine immediately if decompensation occurs. (1)
Serious autoimmune or ischemic disorders — emergency
Symptoms: Symptoms vary by affected organ: chest pain (heart), severe abdominal pain (bowel), vision loss (eye), muscle weakness (myositis), and others.
Action: Report any new or worsening symptoms to your clinician urgently. These conditions can be fatal or cause permanent organ damage. (1)
Bone marrow suppression (severe cytopenias) — emergency
Symptoms: Signs of severe infection (high fever, chills), severe bleeding, extreme fatigue, fainting.
Action: Contact your clinician immediately. Blood tests are critical; dose reduction or discontinuation may be necessary. (1)
Interactions
| Medicine or test | Effect | Action |
|---|---|---|
| Theophylline | Blood levels of theophylline may increase by about 25%, raising the risk of side effects such as nausea, tremor, rapid heartbeat. | Theophylline levels should be monitored and dosage adjusted by your clinician. (1) |
| Methadone | Methadone levels may increase 10-15%, potentially causing toxicity (sedation, slowed breathing). | Watch for signs of methadone overdose and contact your clinician if concerned. (1) |
| Azathioprine | Serious pancytopenia (low red cells, white cells, and platelets) has been reported when used with peginterferon alfa-2a. | Avoid concurrent use. If combined, strict monitoring of blood counts is required. (1) |
| Nucleoside analogues for hepatitis B | Increased risk of certain toxicities; interactions may occur. | Your clinician will monitor for toxicity and adjust doses as needed. (1) |
Pregnancy, breastfeeding, kidney/liver function, age
Children and adolescents
Peginterferon alfa-2a is used in children: for chronic hepatitis B, children 3 years and older with HBeAg-positive non-cirrhotic disease; for chronic hepatitis C, children 5 years and older in combination with ribavirin. Dosing is based on body surface area. Growth inhibition can occur in children treated for hepatitis C (weight percentile decrease). Regular monitoring is essential. (1)
Pregnancy
Peginterferon alfa-2a alone may be used if benefits outweigh risks, but when combined with ribavirin, pregnancy is contraindicated. Women of childbearing potential must have monthly pregnancy tests during treatment and for 6 months after stopping ribavirin. Effective contraception must be used. Do not use ribavirin if your male partner is pregnant. (1)
Breastfeeding
It is not known if peginterferon alfa-2a passes into breast milk. Discuss with your clinician whether to discontinue nursing or the medicine. (1)
People with autoimmune disease
Autoimmune disorders may worsen; those with autoimmune hepatitis must not receive this medicine. Monitoring for new autoimmune conditions is necessary during treatment. (1)
People with liver disease
Patients with decompensated liver disease (Child-Pugh B or C) must not use peginterferon alfa-2a. Those with compensated disease require close monitoring for decompensation. (1)
People with thyroid disease or diabetes
Thyroid function and blood sugar should be monitored before and during treatment; peginterferon alfa-2a can cause hypo- or hyperthyroidism and hyperglycemia or hypoglycemia. (1)
Monitoring
Before starting
- Confirm indication, age, and compensated liver status.
- Baseline blood tests: complete blood count with platelets, liver function, thyroid function, glucose, and pregnancy test if applicable.
- Assess psychiatric history, cardiovascular risk, autoimmune conditions, and eye exam if needed.
- Plan for ribavirin co-administration if treating hepatitis C, including contraception counseling.
During treatment
- Blood tests at weeks 2 and 4, then periodically; more frequent if abnormalities occur.
- Monitor for neuropsychiatric symptoms (depression, suicidal thoughts) at each visit.
- Monitor for signs of infection, bleeding, cardiac events, vision changes, severe abdominal pain, pulmonary symptoms, thyroid dysfunction, and glucose abnormalities.
- Monthly pregnancy testing for women of childbearing potential on ribavirin combination.
- In children, monitor growth (weight and height) regularly.
- Clinician will adjust dose or discontinue therapy based on toxicity or lack of efficacy.
Storage
Store in a refrigerator at 2°C to 8°C (36°F to 46°F). Do not freeze. Keep in original carton to protect from light. Follow specific pharmacy instructions for preparation and disposal.
Professional information
Clinician-facing context only. Use the current local label for precise dosing and monitoring.
All regions
Indication: Chronic hepatitis B or C
Dose summary: Adults: 180 mcg subcutaneously once weekly. Pediatric hepatitis B: 180 mcg/1.73 m² body surface area once weekly (max 180 mcg). Treatment duration: hepatitis B 48 weeks; hepatitis C per regimen.
Renal context: No specific guidance provided in this summary; refer to the full prescribing information. (1)
Hepatic guidance
Contraindicated in hepatic decompensation (Child-Pugh B/C). Use with caution in compensated cirrhosis; monitor liver function closely.
Overdose note
No specific antidote; overdose may worsen adverse effects. Supportive care and monitoring are recommended. Contact a poison center for guidance. (1)
Frequently asked questions
Why do I need weekly injections?
Peginterferon alfa-2a is pegylated, which means it stays in your body longer and only needs to be injected once a week. (1)
Why do I need so many blood tests?
Interferon can affect your bone marrow, liver, thyroid, and blood sugar. Blood tests help your doctor monitor for side effects and adjust treatment if needed. (1)
What symptoms should I report right away?
Contact your doctor immediately if you have new depression or thoughts of self-harm, chest pain, severe abdominal pain, breathing problems, or vision changes. (1)
Can I become pregnant during treatment?
If you are taking ribavirin with peginterferon, you must not become pregnant. Monthly pregnancy tests and effective contraception are required during treatment and for 6 months after stopping ribavirin. If you are only on peginterferon, discuss pregnancy risks with your doctor. (1)
What is the mechanism of action of peginterferon alfa-2a?
Peginterferon alfa-2a is a covalent conjugate of recombinant interferon alfa-2a with a branched bis-monomethoxy polyethylene glycol moiety, prolonging its half-life for once-weekly dosing. It exerts antiviral and immunomodulatory effects. (1)
What is the generic name of Peginterferon alfa-2a?
The generic name is peginterferon alfa-2a. It is sold under brand names including Pegasys. (1)
What class of drug is peginterferon alfa-2a?
Peginterferon alfa-2a is classed as: Pegylated interferon alfa-2a. (1)
References
- PEGASYS (peginterferon alfa-2a) injection — prescribing informationU.S. National Library of Medicine, DailyMed · United States · dated 2023-12 · accessed 2026-08-08 · source 1
- Peginterferon Alfa-2a Injection — patient drug information (MedlinePlus)U.S. National Library of Medicine (MedlinePlus) · US · accessed 2026-08-14 · source 2
- Pegasys — FDA approval record (Drugs@FDA BLA103964)U.S. Food and Drug Administration · US · accessed 2026-08-14 · source 3
- peginterferon alfa-2a — RxNorm normalized drug concept (RXCUI 120608)U.S. National Library of Medicine (RxNorm) · US · accessed 2026-08-14 · source 4
Brand names by country
Single-ingredient peginterferon alfa-2a names are shown separately by country. Product availability, strengths, and supply rules can change; combination products are not included here.
Priority countries
United States
- Pegasys180 ug/mL, 180 ug/.5mL · pharmaand gmbh
Canada
- PEGASYS